What is meant by thermal comfort?
To have “thermal comfort” means that a person wearing a normal amount of clothing feels neither too cold nor too warm. Thermal comfort is important both for one's well-being and for productivity.
Temperature preferences vary greatly among individuals and there is no one temperature that can satisfy everyone. Nevertheless, an office which is too warm makes its occupants feel tired; on the other hand, one that is too cold causes the occupants' attention to drift, making them restless and easily distracted.
Maintaining constant thermal conditions in the offices is important. Even minor deviation from comfort may be stressful and affect performance and safety. Workers already under stress are less tolerant of uncomfortable conditions.
What factors influence thermal comfort?
Thermal comfort is determined by six factors:
- Metabolic rate (of the persons in the room): varies with the number of occupants, and the amount of activity done by occupants (e.g., sitting in a restaurant versus serving the customers).
- Clothing: varies by individual’s choices in clothing or by work requirements (e.g., chemical protective clothing or rain gear).
- Air temperature.
- Radiant temperature: a complex term, but generally described as how the heat transfers between the body and other objects in the area (e.g., radiation is the process by which the body gains heat from surrounding hot objects, such as hot metal, furnaces or steam pipes, and loses heat to cold objects, such as chilled metallic surfaces, without contact with them).
- Air speed (velocity): the rate of air movement.
- Humidity: a general description of the moisture content of the air.
What temperature should an office be?
A general recommendation provided by CSA Z412-00 (R2106) Guideline on Office Ergonomics is that the temperature be maintained at a summertime range of 23-26°C and a wintertime range of 20-23.5°C at 50% relative humidity. In summertime when outdoor temperatures are higher it is advisable to keep air-conditioned offices slightly warmer to minimize the temperature discrepancy between indoors and outdoors.
The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) Standard 55 - 2013 Thermal Environmental Conditions for Human Occupancy offers that these recommended temperature ranges have been found to meet the needs of at least 80% of individuals. Some people may feel uncomfortable even if these values are met. Additional measures may be required. In some situations, legislation may have specific requirements. A list of temperature requirements found in legislation is available in the OSH Answers document Temperature Conditions - Legislation.
What humidity level should an office be?
The ASHRAE Standard 55-2013 uses a graphic comfort zone method that takes into account the factors of relative humidity, humidity ratio, operative temperature, and wet blub temperature with notes on clothing, metabolic rate, radiant temperature, and air speeds. In Appendix F, ASHRAE states “there are no established lower humidity limits for thermal comfort; consequently, this standard does not specify a minimum humidity level.”
CSA recommendations for temperature (above) are listed at an average of 50% humidity.
Relative humidity levels below 20% can cause discomfort through drying of the eyes and mucous membranes and skin. Low relative humidity levels may also cause static electricity build-up and negatively affect the operations of some office equipment such as printers and computers. Relative humidity levels above 70% may lead to the development of condensation on surfaces and within the interior of equipment and building structures. Left alone, these areas may develop mould and fungi. Higher humidity also makes the area feel stuffy.
The Health and Safety Executive (UK) states that a relative humidity between 40% and 70% does not have a major impact on thermal comfort.
What impact does air velocity have?
Air velocity can be created by the air conditioning or ventilation system, and by cold surfaces (e.g., air flowing towards the floor). Thermal comfort is affected by this air movement. Drafts, especially on the head region (head, neck, and shoulders) and leg region (ankles, feet, legs) can cause discomfort.
Generally speaking, temperatures considered in the comfort zone will increase with increased air speed.
What role do the other factors play?
Thermal comfort also depends on the metabolic rates (activities being done), the clothing a person wears, and radiant temperatures of other surfaces.
Metabolic rate (activities) and clothing will vary from person to person, even if every person wears the same clothing and performs the same activity. Where possible, allow individuals to have some control over clothing options and pace of work.
Radiant temperature sources include floors and windows. For example, poorly insulated windows can create a cold area in the winter, and sunshine can create a warm area in the summer. CSA states that humans are most sensitive to warm ceilings, and to cold vertical surfaces such as windows. Floor surface temperatures that are too high or too low, and that are different than air temperatures also contribute to thermal discomfort.
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